doc/announcement.txt: Fix command to run the demo.

Author: techtonik

doc/announcement.txt

@@ -55,7 +55,7 @@ Roundup requires python 2.4 or later (but not 3+) for correct operation.
 
 To give Roundup a try, just download (see below), unpack and run::
 
-    roundup-demo
+    python demo.py
 
 Release info and download page:
      http://pypi.python.org/pypi/roundup